Tests/RS: Add RS context check for RsBalls setAccel()

mRS could be null when lock and unlock screen by pressing power key
several times, it causes RsBalls crash. Add null pointer check to fix.

Change-Id: If37dee8609420daaff17772d5194cad5531f98e6
Author: Yong Chen <yong.a.chen@intel.com>
Signed-off-by: Yong Chen <yong.a.chen@intel.com>
Singed-off-by: Shuo Gao <shuo.gao@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Author-tracking-BZ: 31271